Many embossed web or sheet-type materials can be fabricated by a pair of embossing rolls, wherein each roll has an embossing pattern engraved on the peripheral surface of the roll. The rolls are inter-engaged with each other via their respective embossing patterns at a certain radial depth of engagement. The inter-engaged rolls rotate in opposite directions and impart embossing patterns on both sides of a deformable web or sheet-type material passing between the rotating embossing rolls. The web or sheet-type material becomes deflected and deformed at the points of contact with protrusions of the inter-engaged embossing patterns of the rolls, pushing the web or sheet-type material into recessions of the embossing patterns of the rolls. Upon disengagement of the protrusions and recessions, the embossed material exits the embossing rolls and retains a certain degree of the imparted deformation as a desired embossing pattern.
When the protrusions and recessions of the embossing patterns of the embossing rolls are relatively large (i.e., in the plan view of the peripheral surface of the roll), and/or when clearances between the walls of inter-engaged protrusions and recessions are relatively large, the embossing patterns on the peripheral surfaces of the rolls can be machined by any suitable machining tools, for example, mills, saws, and the like, made of tool steel, carbide or other hard materials. However, when the recessions of the embossing pattern become too small to be machined by the hard tools and/or when inter-engaged embossing patterns need to form substantially small sidewall clearances between the inter-engaged protrusions and recessions, the embossing patterns can be engraved by a laser technique, burning the recessions of the embossing pattern on the peripheral surface of a roll. Examples of the embossing rolls that are typically engraved by the laser burning technique include embossing patterns containing from about 10 to about 1,000 protrusions or recessions per a square inch area (or about 645 square mm area) of the embossing pattern.
A pair of embossing rolls can comprise “matched” or “unmatched” embossing patterns (or a combination thereof). The term “matched” embossing patterns refers herein to a pair of embossing rolls, wherein, when inter-engaged with each other, the protrusions of a first embossing roll are substantially identical in shape and dimensions with the correspondingly inter-engaged recessions of a second embossing roll, and, vice versa, the recessions of the first embossing roll are substantially identical in shape and dimensions with the correspondingly inter-engaged protrusions of the second embossing roll. The matched embossing patterns can be typically accomplished, for example, when a first embossing pattern of a first embossing roll, which has been engraved by a laser-burning technique herein above, is used as a master pattern of a master roll to chemically etch a second embossing pattern in a second embossing roll, matching the first embossing pattern of the first embossing roll.
However, when the embossing patterns need be “unmatched,” (i.e., when the shape and dimensions of the protrusions of a first engraved roll are substantially not identical with that of the corresponding recessions of the second engraved roll, although the corresponding protrusions and recessions are still positioned in registry relative to each other such that they engage) the above described methods can become limited to situations wherein the unmatched parameters are relatively small. For example, a pair of inter-engaged embossing rolls can be provided with a limited side-wall clearance separating the adjacent sidewalls of the correspondingly inter-engaged protrusions and recessions by a means of coating (e.g., electroplating) the protrusions of a laser-engraved pattern of a first roll and then using the laser-engraved roll as a master roll to chemically etch the corresponding recessions of the second roll, thus producing the second pattern of the second roll that will be unmatched with the first pattern of the master roll after the coating is removed and the protrusions are reduced to the originally engraved size. The sidewall clearance achieved by the means of coating is normally limited to about 0.001″ or about 0.025 mm. The limitation is due to the limited thickness of the coating that can be applied to coat the elements of the embossing pattern without deforming the desired shape of the protrusions and recessions, for example, by rounding the sharp edges of the embossing elements and the like.
Therefore, when the unmatched parameters need to be relatively greater than that which can be provided by the thickness of the coating alone, for example, when a larger sidewall clearance than that obtainable by the coating alone is needed between the inter-engaged protrusions and recessions, for example, from above 0.002″ (or about 0.050 mm) to about 0.008″ (about 0.203 mm) or greater such as to about 0.050″ (about 1.27 mm) and/or when the shapes of the inter-engaged protrusions and recessions are substantially different from each other, the rolls can be engraved independently by a laser burning the corresponding embossing patterns on each of the embossing rolls separately.
Unfortunately, the practicalities of laser burning limit the ability to separately burn the embossing patterns of a pair of rolls that would, when brought into engagement with each other, engage uniformly over a substantially entire area of the embossing patterns. These deficiencies resulting from laser burning each of the paired embossing rolls separately from each other, are partially addressed, for example, in U.S. Pat. No. 5,356,364 (column 3, lines 39-54) with respect to another problem related to a need of obtaining a uniform contact between the protrusions and recessions “everywhere on the embossing roll”. As described in the above-referenced patent, such problems sometimes can be tolerated in applications where “a sufficient and substantial number” of desired uniformed engagements between the corresponding protrusions and recessions of the inter-engaged pair of rolls is acceptable to effect an acceptable quality embossed material.
However, such problems often cannot be tolerated when “a substantial number” of uniform engagements is still not sufficient to produce a desired product. For example, when a desired sidewall clearance between the inter-engaged protrusions and recessions of the embossing rolls is not uniform throughout the entire area of the embossing rolls and there are points of engagement having insufficient clearance in order to separate the sidewall of the inter-engaged protrusions and recessions, the points of insufficient clearance can result in material production defects such as pinholes, nips, and other undesired deformities the embossed web material, which can be unacceptable in such web material products as, for example, a storage wrap material that can be used for wrapping food products and can tolerate none or only a limited number of pinholes, in order to efficiently protect the food product or any other product requiring protection from ambient environment. The term “pinhole” refers herein to a through opening in the surface of the embossed web material, having a perimeter of any shape comprising curvilinear, rectilinear or any combination thereof, wherein the minimum dimension of the through opening, measured in any direction within the plane of the web material is from about 0.003″ or about 0.076 mm.
Sometimes, the above deformities resulting from the insufficient sidewall clearance can be reduced for certain material-forming instances, especially when a relatively small sidewall clearance is needed, by employing embossing rolls wherein the embossing pattern of at least one of the embossing rolls is engraved in a resilient material such as a rubber and the like, capable to yield slightly to the web, and thus, less likely to damage the web, as described in the above-referenced U.S. Pat. No. 5,356,364 column 1, lines 61-66. However, in addition to the limitation in the range of the sidewall clearance that can be used in the above method, such resilient materials are often prone to accelerated wear, and can result in undesirable production downtime, which is required to remove the worn roll and to install a new roll.
Therefore, it would be beneficial to provide an apparatus comprising at least a pair of embossing rolls having desired size sidewall clearances between the inter-engaged protrusions and recessions of the embossing rolls—such as from about 0.002″ (about 0.050 mm to about 0.008″ (about 0.203 mm) or greater such as to about 0.050″ (about 1.27 mm)—to avoid defects in the embossed material and machine outages due to production downtime.
It would be also beneficial to provide an apparatus comprising at least a pair of embossing rolls having desired size and shape protrusions and recessions separated by desired sidewall clearances to avoid defects in the embossed material and machine outages due to production downtime.
It would be also beneficial to provide an apparatus comprising at least a pair of embossing rolls having desired size and shape protrusions and recessions separated by desired sidewall clearances, wherein the embossing rolls are capable to engage uniformly with each other over a substantially entire area of the corresponding embossing patterns.
It would be also beneficial to provide a method of producing an embossed material of the present invention, especially for products used for food storage, having sufficient barrier properties for gaseous and liquid transmission—made by the embossing rolls of the present invention—having a substantially reduced number of pinholes or defects related to the lack of the sidewall clearance.